[SI-LIST] Re: Serpentine Traces
- From: Robert Haller <rhaller@xxxxxxxxxx>
- To: ericg@xxxxxxxxxxx
- Date: Tue, 10 Sep 2002 14:19:56 -0400
Eric ,
For the example given, 176 ps/in -> 170 ps/in the difference was
between 5/50 and 5/20 (Line width, /Line Spacing). The delay variations
were even more dramatic at tighter spacings.
To answer Andy's question, I click Info in Allegro - which I believe
measures the center of the line.
Regards,
Bob
--
Robert J. Haller (rhaller@xxxxxxxxxx)
Principal Consultant
Signal Integrity Software Inc.
6 Clock Tower Place, Suite 250
Maynard, MA 01754
Phone: (978) 461-0449, ext 15
Eric Goodill wrote:
> Bob,
>
> What is 'close' in your test board?
>
> -Eric
>
> Robert Haller wrote:
>
>
>>Praveen,
>> This is a good question and has been raised before on the SI-LIST.
>>Serpentine etch is often done to match clock lines or source syncronous
>>data lines.
>>
>>I did bench testing of serpentine lines, in conjuction with simulations.
>>I ran SPICE simulations using field solved 2d models then measured
>>varying serpentine configurations. I examined impedance and propagation
>>delay variations as a function of varying spacing, and signal edge rate.
>>
>>When transmission lines are serpentined 'close' to themselves the
>>propagation delay decreases (The lines got faster). For example if I
>>compare a straight control line and a sepertined line of the same length
>>on the same layer, with all the fixturing nulled the propagation delay
>>went from 176ps/in (straight line) to 170 ps/inch (serpentined line).
>>You can think of the signal taking a shortcut across the distributed
>>self capacitance of the closely serpentined lines.
>>You might think a propagation delay reduction is good, but I also found
>>the variability of prop velocity increased. I believe the variability
>>increased because the capacitive coupling varies based on geometry,
>>while normally the dielectric constant variations are the only source of
>>prop delay variability (for a specific rise time and frequency). The
>>variability is a small effect but is important when doing source
>>syncronous designs.
>>
>>
>>Regards,
>>Bob
>
>
>
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--
Robert J. Haller (rhaller@xxxxxxxxxx)
Principal Consultant
Signal Integrity Software Inc.
6 Clock Tower Place, Suite 250
Maynard, MA 01754
Phone: (978) 461-0449, ext 15
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